Is my clematis in trouble ?

This clematis was planted 3 years ago. It has thrived and has spread vertically up to 6 or 7 metres (with the help of a neigbouring hedge and tree, and has filled the 4m wide trellis horizontally. When cutting back the 6 or 7 metre high growth to get is out of the tree, I found some blackened branches with dried, dying brown leaves. Most of the rest of the plant is okay, but the blackened areas are substantial. What is the problem and what can I do please ?